1. Field of the Invention
This invention relates generally to printing images defined by electrical signals and, more particularly, to the enhancement of black images when rendered at a resolution (e.g., 300 dpi) of color images but printed by a higher resolution print cartridge (e.g., 600 dpi).
2. Description of the Related Art
In some printer systems, an image can contain color regions, isolated black regions, and regions with adjacent black and color regions. The black regions can, for example, include black letters within a color region. Some printers are able to render (process) isolated black text at a resolution of 600 dots per inch (dpi) and print the black dots using a 600 dpi print cartridge. In this case, the resolution of the black text is truly at a 600 dpi resolution.
It is common for such printers to only render color images at a resolution of 300 dpi, upscale the data, and then halftone the image at this higher resolution data for printing with higher resolution capable print systems (higher dpi print cartridges do not increase the resolution of dots rendered at a lower resolution). Rendering the color pixels at 300 dpi is much less processor intensive than rendering the color pixels at 600 dpi. There is little loss in image quality in this method, since most practical color images are stored at less that 300 dpi. There is a slight loss in color text sharpness in this method. It is important to provide a high resolution for black ink due to its high contrast with white paper, so printing black, especially text, at the lower resolution may be objectionable. The term “upscaled” in this application refers to doubling the data in both axis—a very simple 2×expansion vertically and horizontally (or could it be described as replicating horizontally and vertically).
In certain situations, such as where black pixels are adjacent color pixels, the black image is rendered at the same “lower” resolution (e.g., 300 dpi) of the color image. In other words, for those areas where the black regions are adjacent color regions, the mixed region is encoded in the 300 dpi format. This is sometimes referred to as the black pixels being processed through the color pipeline.
The lower resolution black regions (e.g., text), rendered with the color at the color resolution, is upscaled with the color data, and then merged with the higher resolution (600 dpi) black data, such as isolated text. The resulting data-doubled text is referred to as “chunky” text. An example of chunky text can be found in FIG. 1A. For comparison, FIG. 1B is an example of script that has been rendered at 600 dpi.
Thus, even though the printer mechanism has the capability to print black pixels at 600 dpi, the black regions rendered at 300 dpi along with the color regions remain encoded for 300 dpi, and simply doubling the pixels provides no apparent increased resolution.
Additionally, some systems may choose to render an entire page at a lower resolution, say 300 dpi, upscale the image to 600 dpi, and then halftone this image at 600 dpi. The image will render much faster at the lower resolution of 300 dpi, and the halftoning of the upscaled data will result in smooth transitions between colors, and be nearly indistinguishable from 600 dpi rendering. Color text will be compromised in this mode. However, when true 300 dpi black text is upscaled and halftoned at 600 dpi, the pixel replication of the true black data still exhibits low resolution edge characteristics.
Systems for smoothing jagged edges of text are known; however, such systems do not address the situation where black images are rendered by the color pipeline. One smoothing algorithm is referred to as a TES (thermal inkjet edge smoothing) algorithm, described in U.S. Pat. No. 5,650,858, entitled “Pixel Image Edge-Smoothing Method and System,” invented by Mark D. Lund, assigned to the assignee of the present invention and incorporated by reference herein. There is a need for an apparatus and an associated method having the feature that black image regions rendered along with color image regions are resolution-enhanced and printed with a resolution higher than that of the color image regions. Another desired feature of the apparatus and the associated method is the representation of the data groups used in processing the black pixel images in word-length data groups.